Many signs located along the sides of highways and other roads are required to be at least partially retroreflective in order to make the signs more easily detectable and legible, particularly at night, to operators of vehicles approaching the signs. The legend on such signs is typically of a contrasting color to that of the background so as to improve the legibility of the information displayed thereon. A common construction for such signs is to mount retroreflective sheeting used in the face of the sign upon a rigid member, e.g., a supporting panel of aluminum, wood, or other rigid substrate.
During daylight hours, the legibility of such signs is typically optimum under conditions where the sun is substantially to the driver's back or at another angle, e.g., overhead, such that the sign is illuminated without subjecting the driver's eyes to substantial glare. Similarly, at night the legibility of such signs is typically optimum when the predominant lighting is that provided by the headlights of the driver's own vehicle or other vehicles traveling in substantially the same orientation to the sign, e.g., those vehicles traveling in the same direction as, and directly ahead of or behind, the subject driver's vehicle. However, in daytime conditions where the sun and sign are disposed so that the sign is backlit by the sun or where the environment behind the sign is one of bright haze, the face of the sign may be effectively shadowed so as to substantially impair the legibility of the sign. In many instances, the face of the sign may be rendered virtually illegible at useful distances, i.e., those distances at which the legend is to be read, referred to in U.S. Pat. No. 4,726,134 (Woltman) as the legibility zone, even when the legend and background are of substantially contrasting colors, e.g., black letters on white background. Similar interference can occur at night in instances where a sign is backlit by the headlights of oncoming traffic on the same or nearby roads, or by light sources having locations in close proximity with respect to the sign, e.g., street lights. In some instances, a conventional opaque sign may be essentially illegible and be perceived as only a silhouette.